1. Field of the Invention
This invention relates to a color cathode ray tube and more particularly to a color cathode ray tube having an electron gun assembly for focusing and converging three electron beams arranged in line using a single large-diameter electron lens.
2. Description of the Related Art
In ordinary color cathode ray tubes, screen 2 is formed on faceplate 3 of an envelope as shown in FIG. 1. Skirt 3a of a rectangular panel including faceplate 3 is connected via funnel 4 to neck 5 in which electron gun assembly 6 is received. Deflection unit 7 is disposed around the outer surface of the funnel 4 and neck 5. Shadow mask 9 having a plurality of apertures 8 is arranged to face screen 2 with a gap therebetween. Inner conductive film 10 is applied uniformly from the inside wall of funnel 4 to a part of neck 5. Outer conductive film 11 is applied to the outer surface of funnel 4. An anode terminal (not shown) is provided on the funnel 4.
Phosphor stripes or dots are formed on the face plate 3 to form a phosphor screen 2. When the three electron guns beams BR, BG and BB emitted from the electron pass through shadow mask 9 and land guns on the corresponding phosphor spots, the electron-bombarded spots of the phosphor layers emit red, green and blue light rays.
Electron gun assembly 6 includes an electron beam generator GE for generating, accelerating and controlling in-line beams BR, BG and BB and main electron lens section ML for focusing and converging these electron beams. The electron beams BG, BR and BB generated from the electron gun assembly are deflected by deflection unit 7 to scan the whole area of the screen, thereby forming a raster on the screen.
U.S. Pat. No. 2,957,106 discloses an electron gun assembly for converging the three beams on the screen in which the side beams of the three beams are inclined with respect to the center beam and are crossed with the center beam. In addition, U.S. Pat. No. 3,772,554 discloses an electron gun assembly for converging the electron beams in which side apertures are formed on an electrode through which the side beams pass as to have a center which are slightly shifted outwardly from the center axis of the corresponding side electron gun. Thus, the electron beams passing through the side apertures are converged on the screen. Both of these techniques have been adopted extensively in color cathode ray tubes. The deflection unit includes a horizontal deflection coil for generating a horizontal deflection magnetic field to deflect the electron beams in a horizontal direction and a vertical deflection coil for generating a vertical reflection magnetic field to deflect the electron beams in a vertical direction. In the color cathode ray tubes, when the electron beams are deflected, the deflection force causes the three electron beams not to be converged correctly. For this reason, self-convergence magnetic fields are formed, in which the horizontal deflection magnetic field is a pincushioning type and the vertical deflection magnetic field is a barrel type. Also, a convergent free system has been adopted in which the three electron beams can be converged ear the whole area of the phosphor screen.
As mentioned above, the quality of color cathode ray tubes has been improved by the adoption of many newly-developed techniques. However, as larger and higher-grade tubes are manufactured, new problems have arisen. Among these problems is a problem of whether the electron beam spot is formed on the screen with a sufficiently small diameter, a problem of the distortion of the electron beam spots at the peripheral portion of the screen when a beam is deflected thereto and a problem of whether a correct convergence can be achieved in the whole area of the screen. As the cathode ray tube becomes large in size, the distance from the electron gun to the screen becomes longer, the electrooptical magnification of the electron lens becomes large, the beam spot diameter on the screen becomes large, thereby degrading video resolution. To solve this problem, it is necessary to improve the performance of the electron lens of the electron gun so that the beam spot on the screen is made smaller in diameter.
Generally, in the main electron lens section, a plurality of electrodes having openings are arranged along an axis and specified potentials are applied respectively to the plurality of electrodes. There are different types of electrostatic lenses based on different types of electrode construction. To be sure, the lens performance can be improved basically either b forming a large-diameter lens with a large electrode aperture or by forming a long focal-distance lens with gradual changes in potential by increasing the distances between the electrodes. However, since the electron gun of a color cathode ray tube is received in the neck portion, which is generally a thin glass cylinder, the electrode aperture or the lens diameter is limited physically. Further, the distances between the electrodes are limited to prevent the electric field formed between the electrodes from being affected by other undesirable electric fields in the neck.
In the color cathode ray tubes such as shadow mask type in which three electron guns are arranged in a delta or in-line, as described above, as the electron beam spacing Sg is made smaller, the three electron beams can be converged more easily at one point near the whole area of the screen and less electric power is required for deflection. However, in order for the electron guns to be more closely arranged, the electrode aperture has to be decreased.
Therefore, a technical solution is conceivable in which the co-planer three electron lenses are combined to form a large electron lens so that the performance of the large-diameter electron lens can be fully exercised. FIG. 2 illustrates the large-diameter electron lens. As is clear from FIG. 2, the cores of the electron beams formed on the screen are reduced but they do not have correct shapes. In other words, when the three parallel electron beams BR, BG and BB pass through a common large-diameter lens LEL, if the center beam BG is correctly converged as in FIG. 2, the outer beams BR and BB will be overfocused and overconverged and the beam, spots with a large comatic aberration will be formed on the screen. That is to say, the three beam spots SP, SP and SP will be formed on the screen greatly spaced apart from one another and the outer beam spots will be distorted.
In order to match all converged conditions of the three electron beams and reduce the comatic aberration, the mutual spacing Sg of the three beams with respect to the lens diameter D of the electron lens LEL needs to be decreased to some extent. With regard to the focused conditions of the three beams on the screen, it is necessary to minimize the Sg, but there is a limitation to this approach because of the mechanical arrangement of the electron beam generator section.
Japanese Patent Publication No. 49-5591 (U.S. Pat. No. 3,448,316) and U.S. Pat. No. 4,528,476 disclose that. of the three electron beams incident on the electron lens LEL, the side electron beams are inclined by the angle .theta.0 with respect to the center electron beam as shown in FIG. 3, and the three beams pass through the central part of the electron lens LEL at the same time. In this way, all converged conditions of the three beams are matched. The two side beams passing in the directions coming away from the center electron beam emerging from the electron lens LEL are deflected forcibly by the second lens LEL2 by the angle .phi. in the opposite directions. Therefore, the three beams are converged near the screen. Thus, the convergence and the focusing of the three beams are improved. However, there still remains a problem that a great deflection aberration or comatic aberration occurs in the two outer beams.
As described above, it is difficult, by the conventional techniques, to form a large-diameter electron lens that works equally on the three electron beams and utilizes the performance of large-diameter electron lenses to the fullest.
As we have seen, in order to further improve the picture image performance of color cathode ray tubes, it is effective to improve the performance of the electron gun by using a large-diameter electron lens common to the three electron beams. The conventional techniques, however, have their limitations in that they do not allow optional function of large-diameter electron lenses and are not useful in further improving the picture image performance of color cathode ray tube apparatuses. Therefore, to further enhance the performance of the picture image of color cathode ray tubes, it is desirable to develop a color cathode ray tube having an electron gun, capable of allowing a large-diameter electron lens to function optionally.